Wrapping/hiding nlohmann::json implementation

[damiano-lonigo]

Hello,
I am trying to hide the chosen json implementation using a wrapper class.
Normally there is a forward declaration in the .h something like

``
namespace nlohmann { class json; }

class Pjson
{
nlohmann::json *implP;

public:
Pjson();
virtual ~Pjson();
};
``

and then in the .cpp there is

`#include <Pjson.h>
#include <json.hpp>

using Json = nlohmann::json;

Pjson::Pjson()
{
implP = new Json();
}
`

I have a weird error reference to 'json' is ambiguous

is there any way out of it ?

Thank you

[falbrechtskirchinger]

namespace nlohmann { class json; }

This is not a correct forward declaration. Please include json_fwd.hpp which forward declares the nlohmann JSON types correctly.

[damiano-lonigo]

ok, downloaded, also added abi_macros.hpp and now I get json.hpp:19273:7: error: redeclared with 10 template parameters

[falbrechtskirchinger]

You're probably mixing versions or amalgamated and non-amalgamated headers. Download both json.hpp and json_fwd.hpp from the list of release artifacts here: https://github.com/nlohmann/json/releases/tag/v3.11.2
(Note that include.zip doesn't contain json_fwd.hpp. This was an oversight and it will be included in the next release.)

If you want to use the headers from develop choose between the amalgamated headers in the folder single_include or the individual headers in include and don't mix and match.

[falbrechtskirchinger]

ok, downloaded, also added abi_macros.hpp and now I get json.hpp:19273:7: error: redeclared with 10 template parameters

This specific error is due to the upcoming release adding a tenth template parameter.

[damiano-lonigo]

yes, right, correct

when using the correct files, then, it works

just wondering if we are making the whole "programming environment" a bit too fragile

anyway, yes, thank you

[falbrechtskirchinger]

Well, just upgrade these files together, don't forward declare yourself, and everything should be fine.

nlohmann::json is just an alias and if that's the only type you care about, you should treat the nlohmann:basic_json<...> template as a black box and don't worry about it. The only time this would be expected to possibly break is the next major version release.

[damiano-lonigo]

I want to make your answer as a "solution" but I have the "mark answer" button on what I post and not on you post" ....

weird

[falbrechtskirchinger]

🤷‍♂️

Maybe @nlohmann can mark the answer for you.